For a carrier wave carrying a microwave from an MW sensor 1, pulse intervals are varied between when the MW sensor 1 generates an alert and when the MW sensor 1 does not generate an alert. When checking a sensing area, an operator carries a receiver being able to receive a transmitted wave from the MW sensor 1, walks around an area thought to be the sensing area and checks a monitor screen of the receiver. In this way, the sensing area can be checked at hand by the operator.